Embossing tools are used in a wide variety of applications for transferring data from one medium to another. One example is the mass production of holograms, using embossing tools, for the holographic portions of credit cards. Holograms are also created for a large number of other products, including periodical advertising, wrapping paper and other decorations, a wide range security devices, packaging, and artwork. Mass production of such holograms is carried out by impressing a film with an embossing die containing the holographic relief pattern.
The embossing die or tool is the key feature of this process, and requires a high degree of precision in its manufacturing in order to successfully generate large quantities of high quality holograms. Conventionally, such tools are produced using a number of steps. The first step involves a photoresisted plate exposed to a holographic interference pattern used to form the “master” hologram. After exposure to the holographic light pattern, the photoresist is developed to form a surface-relief pattern in the photoresist corresponding to the holographic interference pattern.
Then a thin layer of nickel is electroformed onto the relief surface of the photoresist to transfer the interference pattern. The nickel layer is then peeled away from the photoresist. This nickel layer becomes the master, and a second nickel layer is made as a copy of the first. The second nickel layer servers as the stamper, and is clamped to a substrate, either by being wrapped around the cylinder or being held to a flat plate which forms a die. Each of the layers is known as a shim. Generally there are two shims used, with one being kept aside as a master while the other is used as a stamper. The metal die is then used to stamp or press the transparent film, impressing the holographic interference pattern onto the film, thereby creating the production holograms. This process is used for a wide variety of different types of holograms, including those constituted by translucent films viewed with laser light, defraction patterns, “2D 3D” holograms, and virtually any other holographic pattern that lends itself to mass production.
Unfortunately, there are certain limitations to such conventional techniques. For example, nickel is the preferred metal for creating an embossing tool to transfer the holographic interference pattern. Accordingly, only material softer than nickel can be imprinted by the die. For example, nickel can be used for imprinting on polyester films, soft plastics and the like. However, even with such soft material nickel deforms after continuous use so that after a few thousand meters of embossing, the embossing tool deteriorates to the point that it is no longer suitable for turning out copies.
Also, when a die is wrapped around a printing cylinder or roller, a seam runs approximately parallel to the axis of the cylinder. For the sake of accuracy, the holographic pattern cannot be located anywhere near the seam. In particularly large printing cylinders, multiple shims are used since it is often very difficult to make shims large enough for some embossing rollers. The chief problem with this situation is that there is a mark in the material every time that this seam comes around as a result of the rotation of the roller. As a result, a good deal of the roller surface is lost for practical use, and the size of the hologram that can be printed is severely limited. Since it is very difficult to make a solid cylindrical roller by a metal deposition process, such as electroplating, this problem probably cannot be solved using conventional techniques, which are not adaptable to creating a seamless roller.
Other, harder materials have been tried in the production of embossing tools. For example, aluminum has been used for embossing dies for holographic transfer. However, the process for making the aluminum die is severely limited by the chemical etching process so that accuracy of the etched pattern degrades rapidly with increasing depth of the etching. Also, aluminum is not a particularly good material for extremely high pressures, such as those used in creating embossed holograms.
Other, harder materials have been used. For example ion milling or etching has been used to create holographic patterns on such materials as tool steel, diamond or chromium. One example of such a technique is found in U.S. Pat. No. 5,521,030 to McGrew. This patent discloses the use of anisotropic reactive-ion etching to transfer a holographic interference pattern from a photoresist to the durable material to be used as an embossing tool.
Unfortunately, such techniques for forming holographic interference patterns on durable materials are extremely expensive and complex to carry out. Such techniques are only justified when large numbers of accurate holograms have to be embossed. Otherwise, the expense of such techniques can not be justified. Further, none of the conventional techniques are capable of providing for seamless embossing, or any of the advantages associated therewith. Accordingly, there is still a need to inexpensively create embossing tools on hard material that can be used to transfer holographic interference patterns under conditions of high pressure without degrading the resultant copies, even for extended production runs.